Effect of calcium on neurohumoral stimulation of feline colonic smooth muscle.

The purpose of this study was to compare the effect of altering the extracellular calcium ion concentration on bethanechol or octapeptide of cholecystokinin (OP-CCK) stimulation of the isolated transverse colon of the cat. Myoelectric activity was recorded with monopolar glass-pore electrodes. Bethanechol (10(-6) M) stimulated an increase in the number of slow waves with superimposed spike potentials to 85.5 +/- 5.3% (P less than 0.001) compared with the basal spike activity (8.9 +/- 1.4%). OP-CCK (4 x 10(-9)) also increased spike activity (80.7 +/- 3.8%, P less than 0.001), which was not inhibited by atropine, phentolamine, or propranolol. Addition of 0.0 mM calcium solution to the colonic smooth muscle abolished both slow-wave and spike activity, which returned after replacing 0.25 mM calcium in the solution. Bethanechol stimulated a greater increase in spike activity as the concentration of calcium was increased. OP-CCK stimulation of colonic spike activity was more sensitive to the extracellular calcium concentration than bethanechol stimulation. Verapamil had a minimal effect on bethanechol stimulation of colonic spike activity, but it inhibited the OP-CCK stimulation. These studies suggest that 1) OP-CCK appears to stimulate colonic smooth muscle directly and 2) OP-CCK requires the presence of a greater amount of extracellular ionic calcium in order to stimulate colonic spike activity compared with bethanechol.